Alignment process using serial detection of repetitively patterned alignment marks

ABSTRACT

The specification discloses a process for accurately aligning a chosen member with a focused beam of radiation or for aligning two chosen members with each other. A set of alignment marks having a predetermined spatial pattern is first provided on the surface of each member. Next, a detection signal is generated from the set of alignment marks, comprising a serial electronic signal as a function of time and containing serial information corresponding to the relative position of each mark in the set of alignment marks. Then, the detection signal is electronically processed to generate an alignment signal with a high signal-to-noise ratio. Next, the alignment signal is compared to a reference signal signifying a predetermined criterion of alignment, to generate an error signal which is indicative of the extent of misalignment. Finally, lateral movement of one chosen member is produced in response to the error signal until accurate alignment of the chosen member with the focused beam of radiation or of the two chosen members with each other is achieved.

The Government has rights in this invention pursuant to Contract No.N00123-78-C-0195 awarded by the Department of the Navy.

This application is a continuation-in-part application of patentapplication Ser. No. 149,600, filed May 13, 1980, which has matured toU.S. Pat No. 9,327,292.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to micropattern generation andreplication techniques, and, more particularly, to a novel alignmentprocess for accurately aligning a given object with a focused beam orfor accurately aligning two given objects with each other, by usingserial detection of repetitively patterned alignment marks and specialinformation processing techniques.

2. Description of the Prior Art

In the fabrication of semiconductor integrated circuits and devices, animportant goal has been the increased microminiaturization of thesecircuits and devices in order to produce structures having minimizedsize and maximized component density, speed, and complexity. At variouspoints during such fabrication processes, it is frequently necessary toalign a semiconductor wafer or substrate with a mask or a source ofradiation, or both, for processes such as etching, forming ion-implantedregions, or depositing metal patterns. In order to achieve increasedmicrominiaturization of certain types of integrated circuits, analignment process is needed which will provide very accurate alignmentthat is compatible with high resolution of pattern dimensions.

This accurate alignment becomes extremely important when it is necessaryto overlay patterns, i.e., to align a given sample or target withseveral patterns in succession. For example, a representative processfor forming an integrated circuit might require 10 levels of masking andregistration. Since alignment errors become additive in such asuccession of alignment procedures, the necessity for highly accuratealignment is manifest. In addition, the requirement for high resolutionimposed by submicrometer design rules further emphasizes the criticalityof achieving highly accurate level-to-level registration.

There are two types of precision alignment processes which are ofparticular current interest in the fabrication of microminiaturizedcircuits, i.e., circuits having structures with one or more dimensionsof one micrometer or less. The first such process used a focused orshaped beam of radiation, such as a laser, electron, or ion beam, andscans the beam across an alignment mark. The back-scattered or secondaryradiation that is generated when the primary beam strikes the alignmentmark is collected with a suitable detector. The data from the detectoris processed and analyzed in order to determine the position of the beamwith respect to the alignment mark. Alignment is achieved either byvisually matching an alignment signal with a reference signal on adisplay screen or by using the risetime of the signal produced by thedetection means, such as a photocell or video display system, toimplement a servo control system.

A system of the type just described is set forth in U.S. Pat. No.4,056,730, to D. E. Davis et al. In the process of Davis et al, a squareshaped beam of electrons is scanned over a registration mark on thesurface of a wafer and the back-scattered electrons are detected by adiode detector. The detector then produces a signal which shows a peakwhen the electron beam passes over each edge of the registration mark.This signal is then processed and analyzed to provide an indication ofthe location of the registration marks with respect to the electronbeam. One disadvantage of a process such as that of Davis et al is thatthe resolution, or smallest dimension that the process is capable ofachieving, is limited by both the diameter and other variablecharacteristics of the focused beam. If the same electron beam that issubsequently used for the alignment process was also used to fabricatethe alignment marks, the mark so formed may have an imperfect or fuzzyedge due to variations in beam current and skewing of the currentdensity profile. Thus, a sharp edge on the alignment mark is notpresented for alignment purposes. In addition, these beam imperfectionscause further ambiguity when the beam is scanned across the alignmentmark and thus interfere with accurate alignment. The ability to toleratesuch beam imperfections and alignment mark imperfections is low in aprocess in which it is typically required that the beam be aligned to adimension which is a fraction of the beam diameter. For example, in aprocess in which a target is to be aligned with a pattern on a mark forthe purpose of transferring the pattern to the target, it is oftennecessary to align the beam to within one-tenth of the dimension of aline on the mask.

More specifically, the alignment mark used in such a prior art focusedbeam technique typically consists of simple patterns, for example, across or L-shape, and often has dimensions which are many times largerthan the size of the focused beam. However, in a process which detectsprimarily or only the edge of the alignment mark, the width of the edgeis small with respect to the size of the beam and it is thereforedifficult to locate the edge. The ideal conditions for such a processwould be to have a zero width-edge and an infinitely small beam size sothat when the beam is scanned across the edge, the detected signal wouldshow a sharp rise. However, it is not possible to form an edge with azero thickness. The edge has a finite width which is determined by theprocess used to form it, and this width is usually related to the beamdiameter, for example, being one-half the beam diameter. Thus, it isdifficult to very accurately detect the edge of the alignment mark witha focused beam by such prior art processes. In addition, the actuallocation of the edge of the alignment mark on the target has a degree ofuncertainty because of the uncertainties inherent in the processes usedto fabricate the marks (e.g., forming a mask and etching). Further, inthese prior art processes, there are inherent alignment markirregularities, which produce noise in the detection signal.

Another disadvantage of such a prior art edge-detection alignmentprocess is that the signal produced from the information collected bythe detecting means is not a square waveform with an edge correspondingto an edge of the alignment mark, but rather is a waveform which shows agradual rise and fall. Thus, determination of the exact position of thealignment mark edge is not possible from such a gradually sloped signal.In addition, the signal generated from the information from thedetection means will have variations due to non-uniformity in the beam,such as fluctuation in current and skewing of the current densityprofile. Furthermore, there are problems associated with the electricalnoise present in the detector means, which give rise to the necessity ofincreasing the signal-to-noise ratio in order to enhance the signal.Excessive low-pass filtering of the signal to remove noise degrades therisetime and results in no net increase in accuracy.

One approach to increasing the signal-to-noise ratio of the detectedsignal in an edge-detection alignment process is set forth in U.S. Pat.No. 4,123,661, to Edward D. Wolf and Walter E. Perkins, Jr., assigned tothe present assignee, in which high atomic number metals or compoundsare used as electron beam registration alignment marks on low atomicnumber substrates in order to enhance the secondary and back-scatteredelectron video signals. In addition, it may be noted that in the processof Wolf et al, the enhanced signal contrast so produced is furtheraugmented by placing two registration marks sufficiently close together(e.g., at a distance of less than 3 micrometers) that the separationtherebetween will provide sharply definable maximum dip and rise slopesin the video signals, which reflect the position of the edges of thealignment marks. While the process of Wolf et al represents asignificant improvement over the prior art, it still relies on edgedetection for alignment purposes.

Thus, in an edge-detection system, determination of the exact edge ofthe alignment mark to an accuracy that is a small fraction of therisetime of the alignment signal is difficult and is further complicatedby the presence of noise. Finally, in the case where a scanning focusedion beam is used, the ion beam causes sputtering or removal of materialfrom the edge of the alignment mark as it scans over it, which leads toa degradation in edge definition of the alignment mark and consequentdeterioration in alignment accuracy.

The second type of precision alignment process of particular currentinterest in the fabrication of microminiaturized circuits and devicesuses a matched pair of patterned masks on two separate objects, forexample, a mask and a target. A flood of radiation is passed through thefirst set of alignment marks toward the second set of alignment. Whenthe radiation strikes the second set of alignment marks, it causeseither reflected radiation or a secondary emission of radiation from themarks, which is then detected by a suitable detector means. The amountof radiation striking the second set of alignment marks is proportionalto the extent of alignment of the first and second sets of alignmentmarks. When the two sets of alignment marks are perfectly aligned, themaximum amount of radiation will strike the second set of marks. Theinformation from the detector means is processed to produce a signalwhose magnitude is related to the amount of alignment of the first andsecond alignment mark patterns. Such a system using x-rays is set forthin U.S. Pat. No. 3,984,680 to Henry I. Smith.

One disadvantage of such a patterned mark alignment process is that themaximum resolution achievable is limited by the resolution of an elementin the pattern of the alignment mark. In addition, there are frequentlyimperfections in both sets of alignment mark patterns which are inherentin the process steps by which these patterns are formed (e.g., forming aresist pattern, and etching). Thus, when alignment of these twoimperfect patterns is attempted, the waveform of the signal produceddoes not have a perfect maximum, but rather is a "flat" maximum (i.e., asignal with a truncated or flat top). This flat maximum leads toambiguity in determining the maximized reflected signal and thus theoptimized alignment between the two sets of alignment marks. Forexample, if the two sets of alignment mark patterns are visuallyaligned, the patterns may appear fuzzy and the exact edge of thepatterns cannot be determined.

In a related alignment technique which uses matched sets of patternedalignment marks in the form of diffraction gratings, both sets ofalignment marks are illuminated with radiation, such as a laser beam, asdescribed, for example, by D. C. Flanders, H. I. Smith, and S. Austin inthe publication entitled "A New Interferometric Alignment Technique", inApplied Physics Letters, Vol. 31, No. 7, Oct. 1, 1977, pp. 426-428. Theradiation diffracted from each set of alignment marks is detected,corresponding detection signals are generated, and an opticalinterference pattern is generated from the detection signals as a meansof comparing the position of the two sets of alignment marks. An errorsignal is then generated which indicates the extent of alignment of thetwo sets of alignment marks with respect to each other. However, thereare potential systematic errors affecting the accuracy of the alignmentprocess, such as the partial dependence of the magnitude of the errorsignal on the spacing between the grating marks and the possiblenon-symmetry in the profile of the grooves of the grating as formed. Inaddition, the diffraction gratings must be formed to have a frequencywhich is reasonably close to that of the laser light and the spatialfrequency of the marks must be uniform or else multiple beams ofdiffering intensities are formed rather than a collimated, sharplydefined beam as is desired for alignment purposes. Further, thediffraction grating marks must have optical contast in order to beeffective and must have a structure with a periodicity that isaccurately controlled to a fraction of a wavelength of the radiationused. In some cases, these optical and structural requirements may notnecessarily be compatible with semiconductor processing techniques whichwould be necessary to fabricate a semiconductor device. In addition,rotational misalignment of the two diffraction grating marks causesimperfect meshing of the interference pattern and leads to an errorsignal that has a soft maximum. Further, the separation between the twodiffraction gratings must be accurately controlled or else spuriousresults are obtained. Finally, such a diffraction grating system ismechanically complex, requiring, for example, special mirrors andspecial alignment procedures therefor, which gives rise to systematicerrors.

It is the alleviation of many of the above-mentioned problems in priorart alignment processes to which the present invention is directed. Moreparticularly, some of the problems that the present invention seeks toovercome are: the limitations on maximum achievable resolution imposedby process parameters such as beam diameter or resolution of an elementof the alignment mark pattern; the inability to accurately detect theedge of an alignment mark; and the inability to generate an accuratealignment signal with a high signal-to-noise ratio.

SUMMARY OF THE INVENTION

The general purpose of this invention is to provide a new and improvedalignment process for aligning a chosen member with a focused beam ofradiation or for aligning two chosen members with each other, which ishighly accurate and provides high resolution, and which possesses most,if not all, of the advantages of the above prior art alignmentprocesses, while overcoming many, if not all, of the above-mentionedlimitations.

The above general purpose of this invention is accomplished in a processfor aligning the surface of a first chosen member with a chosen objectby first providing a set of alignment marks having a predeterminedspatial pattern, on the surface of the chosen member. A detection signalis generated from the set of alignment marks and comprises a serialelectronic signal as a function of time and containing serialinformation corresponding to the relative position of each mark in thealignment mark set. The detection signal is electronically processed togenerate an alignment signal with high signal content pertinent to thelocation of the set of alignment marks. A reference electrical signal isprovided which has a predetermined waveform, corresponding to thewaveform of the alignment signal, and signifies a predeterminedcriterion of alignment. Then, the waveform of the alignment signal iscompared to the waveform of the reference signal to generate an errorsignal which is a function of the time displacement between thewaveforms of the alignment and reference signals. Finally, lateralrelative movement between the chosen member and the chosen object isprovided in response to the error signal until accurate alignment of thechosen member with the chosen object is achieved. The chosen object maybe, for example, a focused beam of radiation or a second chosen member.

More particularly, by a first process embodiment of the presentinvention a chosen member is aligned with respect to the scan field of afocused beam of radiation by: (a) providing a set of fine alignmentmarks having a predetermined periodicity on the surface of the chosenmember; (b) scanning the focused beam over the set of alignment marks ata predetermined and fixed velocity to generate a serial detectionelectrical signal that is indicative of the periodicity of spacing ofthe marks in the alignment mark set and whose phase angle corresponds tothe relative position of the set of alignment marks; (c) electronicallyprocessing the detection signal to generate an alignment signalcomprising a fundamental periodic signal of a selected frequency; (d)providing a reference electrical signal comprising a periodic signal ofthe selected frequency and a predetermined phase which has apredetermined relationship to the scan field of the focused beam; (e)comparing the phase angle of the alignment signal and the phase angle ofthe reference signal and generating an error signal which isproportional to the phase difference of the alignment signal and thereference signal; and (f) providing lateral relative movement betweenthe focused beam scan field and the set of alignment marks in responseto the error signal until the alignment signal and reference signalbecome congruent in phase and the chosen member is accurately alignedwith the focused beam of radiation.

In a second process embodiment of the present invention, a process isprovided for aligning the surface of a first chosen member with thesurface of a second chosen member by: (a) providing a first set ofalignment marks on the surface of the first member, with the marks beingspaced at a predetermined periodicity; (b) providing a chosen fixedreference point for determining the relative positions of the first andsecond chosen members; (c) generating from the first set of alignmentmarks a first detection signal comprising a serial electronic signal asa function of time and containing serial information corresponding tothe position of the first set of alignment; (d) electronicallyprocessing the first detection signal to generate a first alignmentsignal with a high signal-to-noise ratio; (e) providing a firstreference electrical signal comprising a periodic signal of a selectedfrequency and phase: (f) comparing the phase angle of the firstalignment signal and the phase angle of the first reference signal andgenerating a first error signal proportional to the phase difference ofthe first alignment and first reference signals; (g) providing lateralmovement of the first set of alignment marks with respect to the chosenreference point, in response to the first error signal until the firstalignment signal and first reference signal become congruent in phaseand the first chosen member is accurately aligned with the chosenreference point. Then, the second chosen member is similarly alignedwith the chosen reference point by: (h) providing a second set ofalignment marks on the surface of the second chosen member which issubstantially identical to the first set of alignment marks; (i)generating from the second set of alignment marks a second detectionsignal comprising a serial electronic signal as a function of time; (j)electronically processing the second detection signal to generate asecond alignment signal; (k) providing a second reference signal; (l)comparing the phase angles of the second alignment signal and the secondreference signal to generate a second error signal proportional to thephase difference of the second alignment and second reference signals;(m) providing lateral movement of the second set of alignment marks withrespect to the chosen reference point in response to the second errorsignal until the second alignment and second reference signals becomecongruent in phase and the second chosen member is accurately alignedwith the chosen reference point and thus with the first chosen member.

In a third process embodiment of the present invention, yet anotherprocess is provided for aligning the surface of a first chosen memberwith the surface of a second chosen member by: (a) providing a first setof alignment marks on the surface of the first member and spaced at afirst predetermined periodicity; (b) providing a second set of alignmentmarks on the surface of the second member and spaced at a secondpredetermined periodicity; (c) providing a chosen fixed reference pointfor determining the relative positions of the first and second chosenmembers; (d) generating from the first and second sets of alignmentmarks a composite detection signal comprising a serial electronic signalas a function of time and containing serial information corresponding tothe position of the first set of alignment marks and of the second setof alignment marks; (e) electronically processing the compositedetection signal to generate two alignment signals, one representing theposition of each set of alignment marks; (f) providing first and secondreference electrical signals comprising periodic signals ofpredetermined frequency and phase; (g) comparing the phase angle of thefirst alignment signal with the phase angle of the first referencesignal to generate a first error signal proportional to the phasedifference of the first alignment signal and the first reference signal;(h) providing relative lateral movement of the first chosen member withrespect to the chosen reference point in response to the error signaluntil the first alignment signal and the first reference signal becomeconqruent in phase, to produce accurate alignment of the first chosenmember with the chosen reference point; (i) comparing the phase angle ofthe second alignment signal with the phase angle of the second referencesignal to generate a second error signal proportional to the phasedifference of the second alignment signal and the second referencesignal; and (j) providing relative lateral movement of the second chosenmember with respect to the chosen reference point in response to thesecond error signal until the second alignment signal and the secondreference signal become congruent in phase, to produce accuratealignment of the second chosen member with the chosen reference pointand thus with the first chosen member.

Accordingly, it is an object of the present invention to provide a newand improved highly accurate alignment process of the type describedwhich is capable of providing high resolution.

Another object is to provide a new and improved alignment process forvery accurately aligning a chosen member with a focused beam ofradiation or particles.

Still another object is to provide a new and improved alignment processfor very accurately aligning a first chosen member with a second chosenmember.

Another object is to provide a process for accurately aligning twochosen members with each other by aligning each member with a chosenreference point, either in sequence or simultaneously.

Yet another object is to provide a process of the type described inwhich a periodic or encoded serial electronic signal as a function oftime is generated from a spatial alignment mark.

Another object is to provide a process of the type described in whichpoint information for each alignment mark in a set is generated.

Another object is to provide a process of the type described in which adetection signal is specially processed to provide a signal with a highsignal-to-noise ratio which permits positioning of the alignment marksto a fraction of a resolution element.

Still another object is to provide a process of the type described inwhich the negative effects of localized surface defects andirregularities in alignment marks are minimized.

Another object is to provide a process of the type described in whichlocalized sputtering of an alignment mark by a scanning beam isminimized.

The foregoing and other objects, advantages, and features of the presentinvention will become more readily apparent from the following moreparticular description of the preferred embodiments of the invention asillustrated in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram representation of one suitable system usefulfor practicing one process embodiment of the present invention in whicha chosen member is aligned with a focused beam of radiation.

FIG. 2 is a schematic diagram of a one-axis alignment target forcarrying out one process embodiment of the present invention.

FIG. 3 is a schematic diagram of a two-axis alignment target forcarrying out one process embodiment of the present invention.

FIG. 4 is a block diagram representation of one suitable system usefulfor practicing second and third process embodiments of the presentinvention in which a first chosen member is aligned with a second chosenmember.

FIG. 5 shows, in schematic form, the detection and alignment signalsgenerated in practicing one process embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows, in block diagram form, the stages in one suitable systemuseful for practicing one process embodiment of the present invention inwhich a chosen member is aligned with a focused beam of radiation. Achosen member 10, such as a wafer of a selected semiconductor material(e.g. silicon), is placed on the surface of a platform or stage 12. Thewafer 10 of semiconductor material may optionally have layers of othermaterials, such as oxide or photoresist, formed thereon. The wafer 10may optionally be formed of an optical material, such as lithiumniobate.

A set of alignment marks 14 is formed at a predetermined location on thesurface of the member 10, for example, at the peripheral edge thereof,as shown in FIG. 1. The set of alignment marks 14 has a predeterminedspatial pattern and may be in the form of a repetitive pattern of finebars which form a grating structure as described in FIG. 2 or in theform of a two-dimensional array of fine dots of uniform size asdescribed in FIG. 3. Optionally, the set of alignment marks may beformed to have a coded periodicity, such as a Barker code, whosestructure, function, and use is described in the book entitled "RadarSignals, An Introduction to Theory and Application", by Charles E. Cookand Marvin Bernfeld, Academic Press, New York, 1967, Chapter 8--DiscreteCoded Waveforms, at page 226 et seq. When using a set of alignment markssuch as those shown in FIGS. 2 or 3, the periodicity of the alignmentmarks 14 is typically chosen to be equal to the diameter of the spotsize of the focused beam 16 in order to maximize the alignment accuracy.Since displacement of the alignment mark pattern by a distance equal tothe period of the pattern spacing would produce ambiguity in determiningthe exact location of the alignment mark, it is necessary to include ameans for achieving coarse alignment of the alignment mark as apreliminary procedure, as discussed in relation to FIGS. 2 and 3.

Spaced at a predetermined distance above the member 10, there is a beamsource 18 which produces a focused beam of particles or radiation 16that strikes the alignment marks 14 and the portion of the surface ofmember 10 adjacent thereto. The beam source 18 may produce an ion beam,an electron beam, or a laser beam. As a practical matter, if possible,the beam source 18 for the alignment process described herein would beidentical to the beam source used in the further processing steps (e.g.,ion implantation) to be performed subsequent to the alignment process,in order to simplify equipment requirements. An ion or electron beam isfocused as described, for example, in the book entitled "ElectronOptics", by P. Grivet, Pergamon Press, New York, 1965, pp. 3-257.Adjacent to the focused beam 16, there are conventional beam deflectors20, which are controlled by a deflection signal generator means 21 andwhich are used to deflect the focused beam 16 and thus control itsposition. The beam deflectors 20 comprise conventional electrostatic ormagnetic structures which apply the appropriate force to deflect thefocused beam 16 in a desired direction. The deflection signal generatormeans 21 provides a deflection signal to the beam deflectors 20, vialines or conductors 19, and this deflection signal determines theconfiguration of the selected region that is scanned by the focused beam16. Such a deflection system for ion or electron beams is described, forexample, in the book entitled "Theory and Design of Electron Beams", byJ. R. Pierce, D. Van Nostrand Company, Inc., 1954, Chapter III. SimpleElectron Motions.

When the focused beam 16 strikes the alignment marks 14, it causessecondary emissions 22 from the alignment marks 14 to be produced. Forexample, if the focused beam 16 comprises a beam of ions, the secondaryemissions 22 produced from the alignment marks 14 comprise secondaryelectrons. By detecting the secondary electrons produced by impingementof the beam on the alignment mark rather than detecting theback-scattered electrons as in the prior art processes previouslydiscussed, this embodiment of the present invention offers the advantagethat ion beams (which produce only secondary electrons) may be used asthe radiation source. In addition, when using an electron beam source inpractising the present invention, a higher yield of detected electronsmay be achieved for lower quantum noise than by some prior art processespreviously discussed. If the focused beam 16 comprises a beam ofelectrons, the secondary emissions 22 consist of back-scattered primaryelectrons. If the focused beam 16 comprises a laser beam, the secondaryemissions 22 comprise secondary electrons or scattered photons.

The secondary emissions 22 are detected by a detector means 24, whichmay be, for example, a video display system comprising an electrondetector, a high bandwidth amplifier, an X-Y raster deflectionsynchronized to the primary focused beam deflection, and a cathode raytube display in which the intensity is modulated by the detectedelectron current. The detector means 24 produces an electrical outputsignal, referred to herein as a detection signal, which is a serialelectronic signal that is indicative of the periodicity of spacing ofthe set of alignment marks and whose phase angle corresponds to therelative position of the set of alignment marks with respect to thescanfield of the focused beam. The detection signal passes via line orconductor 25 to the special signal processing means 26, which comprisesnarrow bandpass filtering and tuned amplification means and is discussedin more detail below.

The detection signal is passed through a narrow bandpass filter, whichfilters out noise and passes through the signal at a selected frequencywhich has previously been determined from Equation (1) below. ##EQU1##where f_(bp) =center frequency of bandpass filter;

scan velocity=velocity of beam scanning the alignment marks;

line spacing=distance between adjacent alignment marks in a set.

One advantage of the present invention is that for the type of detectionsignal produced, the optimum filter to reject random noise is a simplebandpass filter. Such filtering is especially useful when the alignmentmark has a low signal-to-noise ratio, which could occur when thedetection signal is produced by detecting secondary electrons fromrelatively shallow profile alignment marks.

From the narrow bandpass filter, the signal is passed through anamplifier which amplifies only the signal at the previously selectedfrequency. To be effective for the purposes intended herein, the tunedamplifier would preferably have low input noise. Thus, by thecombination of filtering the detection signal to obtain a signal of aselected narrow bandwidth and then amplifying this signal only at aselected frequency, an alignment signal is thus produced as afundamental periodic signal of a predetermined frequency. The signalproduced by this special signal processing means 26 is referred toherein as an alignment signal.

A reference signal generator 28, comprising conventional andcommercially available electronic circuitry, generates a referencesignal having a predetermined waveform that approximates the waveform ofthe detection signal. Using the set of alignment marks shown in FIG. 2in this first process embodiment of the present invention, the detectionsignal is in the approximate form of a sine wave. The reference signalgenerated is a sine wave signal of predetermined frequency and phase.The frequency of the reference periodic signal is selected so that onefull period of the signal is generated for each mark in the alignmentmark set 14. The phase of the reference signal is adjusted to have apredetermined relationship to the scan field of the beam 16, and, forexample, is adjusted and so that the beginning of the reference signalis synchronized with the beginning of the deflection signal generated bythe deflection signal generator 21, optionally referred to as a scangenerator. More particularly, the sawtooth, or repetitive ramp, signalwhich is indicative of the manner in which the focused beam 16 isdeflected, is used as the trigger to start the reference signal, asindicated by legend on signal path 22 in FIG. 1. (The step signalresulting from the differentiation of the ramp signal to form thederivative thereof is the trigger for the reference signal to start.)The frequency of the reference signal generator 28 is thenelectronically adjusted to equal the frequency of the alignment signaldiscussed above, as indicated by the legend on signal path 27 shown inFIG. 1.

The reference signal need not be in the form of a sine wave, but maytake the form of another periodic or encoded signal. If the set ofalignment marks is formed to have a selected coded periodicity such as aBarker code, from which corresponding detection and alignment signals inBarker code are generated, the reference signal generated would be acorresponding encoded signal.

The reference signal generated by the reference signal generator 28 isinput via line or conductor 29 to a comparator means 30. The alignmentsignal from the special signal processing means 26 is also input to thecomparator means 30 via line 31. The comparator means 30 compriseselectronic circuitry which compares the phase angles of the alignmentand reference signals applied thereto and generates an error signalwhich is proportional to the phase difference of these two signals,which, in turn, is indicative of the extent of misalignment of thechosen member 10 with the scan field of the focussed beam 16.

If the detection signal and the alignment signal derived therefrom andthe reference signal are in a code such as a Barker code, the comparisonof the alignment and reference signals is performed by a knowncorrelation technique to generate an error signal which reflects theextent of correlation of the alignment and reference signals in time.

The error signal from the comparator means 30 is subjected to low-passfiltering to improve the signal-to-noise ratio and is then input vialine 33 to a means for providing lateral relative movement between thescan field of the focused beam 16 and the chosen member 10. For example,as shown in FIG. 1, the error signal from the comparator means 30 may besent via line 33 to a stage controller 32, which generates drive signalsapplied via line 35 to a transducer means 34, which, in turn, causesmechanical motion of the stage 12 holding the wafer 10 by means of asignal on line 37 in a direction and magnitude determined by the errorsignal, in a manner similar to that described, for example, in U.S. Pat.No. 4,019,109 to John H. McCoy and Paul A. Sullivan, and in U.S. Pat.No. 4,109,029 to Faik S. Ozdemir and Dall D. Loper, both assigned to thepresent assignee and both incorporated herein by reference with allreferences cited therein.

Optionally, the error signal from the comparator means 30 may be inputto a DC offset circuit (not shown), which provides a signal to the beamdeflectors 20 to shift the position of the focused beam 16 in adirection and magnitude determined by the error signal. Such a beamdeflection system is described, for example, by J. R. Pierce previouslyreferenced.

It should be noted that when an ion beam or electron beam is used, thebeam source 18, the beam deflectors 20, the detector means 24, the wafer10, and usually the stage 12 are all enclosed within a high vacuumchamber indicated by the dashed line in FIG. 1, as is normally requiredfor the functioning of a focused ion or electron beam system.

In practicing the present invention in accordance with the processembodiment as illustrated in FIG. 1, and using the set of alignmentmarks shown in FIG. 2, a coarse alignment procedure is first performedas discussed in relation to FIG. 2. The beam source 18 is used toproduce the focused beam 16, using the beam deflector means 20 asrequired to position the focused beam 16 above the alignment marks 14 onthe chosen member 10. The focused beam 16 is continuously scanned backand forth across the set of alignment marks 14 at a predetermined andfixed velocity, which is determined, in part, by the maximum velocitythat is consistent with system constraints, and is typically within afrequency range of 1 kilohertz to 1 megahertz. The focused beam 16 israster scanned over the entire set of alignment marks so thatimpingement of the beam is evenly distributed over the entire set ofalignment marks 14 and over the entire length of each mark in the set.As the focused beam 16 strikes each alignment mark in the set 14 insequence, it causes secondary emissions in quantity as a function oftime, and more specifically, as a function of the scanning speed of thefocused beam 16.

The detector means 24 detects the secondary emissions as a function oftime and generates a serial electronic signal as a function of time andcontaining serial information corresponding to the position of each markin the set of alignment marks 14.

Thus, by the process of the present invention, spatial informationconcerning the position of the alignment marks 14 is converted into atime periodic signal. This periodic signal contains serial informationon the position of each mark in the set of alignment marks 14. Inaddition, because a plurality of fine lines is used in the alignmentmarks of the present invention, the alignment mark set has a high signalcontent to indicate the position of the alignment mark set. Thealignment mark pattern used in the prior art edge-detection methodpreviously discussed usually presents only two edges in each alignmentdirection, whereas the multiple bar pattern used in the presentinvention contributes far more positional information in a given scanframe than does a single bar. Because of the abundance of this pointinformation and high signal content pertinent to the location of the setof alignment marks, it is possible by the process of the presentinvention to determine the position of the set of alignment marks 14with a much higher degree of accuracy than the prior art edge-detectionprocess previously discussed, in which only information reflecting theposition of the edges of the alignment marks is used for alignment.

In addition, the use of a set of alignment marks 14 with finedimensions, e.g. 0.1 to 0.2 micrometers, further enhances the highresolution of the process of the present invention. For example, in aprior art edge-detection process of the type previously described,achievable resolution is reported to be on the order of the width of therisetime of the video signal used for alignment, or 0.25 micrometers. Bythe process of the present invention, the resolution achievable is atleast 0.05 to 0.10 of the nominal spacing between the alignment marks inthe set, which corresponds to approximately 0.1 of the beam diameterused to form the alignment marks, as previously discussed. For theembodiment illustrated in Example 1, the resolution achievable is 50 to100 angstroms.

The electrical signal from the detector means 24 is subjected to specialsignal processing, as previously described in detail, to generate afundamental periodic signal of a specified frequency, which is used asan alignment signal. This alignment signal may be, for example, in theform of a sine wave as shown in FIG. 5, which is capable of beingaccurately compared in stage 30 to the reference signal sine wave. Thus,by the process of the present invention, the accuracy of the alignmentprocess is significantly improved by producing an alignment signal ofhigh signal-to-noise ratio.

The alignment signal from the special signal processing means 26 iscompared to the reference signal from the reference signal generator 28by the comparator means 30, to produce an error signal which isproportional to the phase difference of the alignment signal and thereference signal. This error signal is filtered and is then used tocause lateral movement of either the chosen member 10 or the scan fieldof the focused beam 16 until the alignment signal and reference signalbecome congruent in phase, indicating accurate alignment of the chosenmember and the scan field of the focused beam.

Thus, by the process of the present invention as illustrated in FIG. 1,it is possible to very accurately align a chosen member, such as asemiconductor wafer, with the scan field of a focused beam and toposition the chosen member to a fraction of a resolution element. Thisaccurate alignment and high resolution made possible by the presentinvention are achieved by generating a serial electronic signal as afunction of time which contains serial information corresponding to theposition of each mark in the alignment mark set. Acquisition of thissignal is then maximized by filtering the signal through a narrowbandpass filter which very effectively removes noise with negligibleloss of signal, and which averages out irregularities in the detectionsignal. In addition, this periodic signal has a high signal contentsince numerous marks in the set of alignment marks are detected.Consequently, a signal with a high signal-to-noise ratio and uniformityis produced for alignment purposes so that the chosen member can bepositioned to a fraction of a resolution element.

In addition, the distributed nature of the alignment informationprovided by the process of the present invention reduces the effect oflocalized surface defects on the alignment marks, such as contaminationor insufficient smoothness of the surface, and resulting irregularitiesin the alignment pattern, and consequently, provides increased alignmentaccuracy and increased alignment reproducibility. Further, since thefocused beam is raster scanned over the total surface of the alignmentmarks, localized erosion of the alignment marks by a scanning focusedion beam is minimized and the lifetime of the alignment marks ismaximized.

In FIG. 2, there is shown a one-axis alignment target for use in oneprocess embodiment of the present invention. This target comprises arepetitive pattern of fine bars 50 which form a grating-like structure.The aggregate of bars 50 is referred to herein an an alignment mark setor as a set of alignment marks. Each bar 50 is referred to herein as amark in the alignment mark set. In order to optimize the alignmentaccuracy, the periodicity of the grating structure and the width of eachbar 50 are chosen preferably to be equal to the spot size of the focusedbeam 16 in FIG. 1 or the minimum video resolution element for the systemin FIG. 4.

Since displacement of the grating pattern by a distance equal to theperiod of the grating spacing results in ambiguity in position, it isnecessary to have a coarse alignment bar 52 at the periphery of thegrating structure comprising bars 50. A preliminary, coarse alignment ofthe beam and the alignment mark set to within one period of the spacingbetween the alignment marks in the set is first performed using thecoarse alignment mark 52 by performing a procedure in accordance with,for example, the process of Davis et al, discussed in the prior artsection herein. Then, the fine alignment bars 50 are used as describedin relation to FIG. 1, for example, to accomplish the process of thepresent invention. Optionally, if the set of alignment marks is formedwith a coded periodicity, such as a Barker code, then a coarse alignmentprocedure is not required prior to the fine alignment process of thepresent invention. The alignment target shown in FIG. 2 is useful whenalignment need be performed only along one axis.

These alignment marks 50 and 52 may be formed by depositing a layer of aknown electron beam resist material on the surface of the wafer,exposing the resist layer to a beam of electrons in a predeterminedpattern corresponding to the alignment marks, and using the patternedresist as a mask for chemical or ion beam etching of the pattern intothe wafer. Etching is performed to a depth that is consistent with boththe requirements of the present invention and the constraints of timerequired to perform the etching process. Typical dimensions of a bar 50for use with an ion or electron beam are 0.1 to 0.2 micrometer wide,approximately 100 micrometers long, and 100 to 1000 angstroms deep. Iflonger wavelength radiation, such as visible light, is used, thealignment mark dimensions must be correspondingly larger. As a practicalmatter, it is desirable to minimize the amount of wafer space which isdedicated to the alignment marks.

Optionally, the alignment marks may be formed as raised marks on thesurface of the wafer by depositing a layer of a material such astantalum, aluminum, gold, plastic, or wafer material and forming thedesired pattern therein by known lithographic procedures. The alignmentmarks used in the present invention, may be left bare or may be coatedwith a layer of oxide or a layer of resist as a result of furtherprocessing steps to which the wafer is subjected after a first alignmentprocedure in accordance with the process of the present invention. Inprior art processes such as those previously discussed, degradation ofthe alignment marks results from deposition of a layer of resist oroxide thereon since the edge of the covered mark is less readilydetected than the uncovered mark. Since the process of the presentinvention, on the other hand, is inherently a more accurate processusing alignment signals with high signal-to-noise ratio, these degradedalignment marks can still be accurately detected.

In FIG. 3, there is shown, in schematic form, a two-axis alignmenttarget for use in the process of the present invention. This targetcomprises a two-dimensional array of fine dots 54 of uniform size. Thisarray of dots 54 is referred to herein as a set of alignment marks. Eachdot 54 is referred to herein as a mark in the alignment mark set. Thediameter of each dot and the separation between adjacent dots aretypically chosen to equal the spot size of the focused beam 16 shown inFIG. 1 or the minimum video resolution element for the system shown inFIG. 4, in order to optimize alignment accuracy. The distance betweenadjacent dots 54 does not necessarily have to equal the dot diameter. Toavoid ambiquity caused by displacement of the pattern by a distanceequal to the period of the dot spacing, a larger, coarse alignmentpattern 56 is added to the periphery of the fine alignment dots 54.Thus, a coarse alignment process is first performed using coarsealignment pattern 56 and, for example, an edge-dection alignment processin a manner similar to that described in relation to FIG. 2, and then afine alignment process in accordance with the present invention isperformed using the fine alignment dots 54. The alignment pattern 56 anddots 54 shown in FIG. 3 may be formed in a manner similar to the processfor forming the bars 50 and 52 described in FIG. 2.

Directing attention now to FIG. 4, there is shown, in block diagramform, the stages in one suitable system useful for practicing second andthird process embodiments of the present invention in which a firstchosen member is aligned with a second chosen member. A first chosenmember 60, such as a wafer of a selected semiconductor material, isplaced on the surface of a stage or platform 62. On the surface of thefirst chosen member 60, there is formed a first set of alignment marks64, which may be of the type shown in FIG. 2 or FIG. 3 or may have acoded periodicity.

In accordance with the second process embodiment of the presentinvention, the second chosen member 66 shown in FIG. 4 is not presentinitially. The first chosen member 60 is first aligned with respect to afixed reference point. Then, the second chosen member 66 is aligned withrespect to the same reference point. Because this second processembodiment of the present invention can be more readily illustrated byshowing the alignment of the second chosen member 66, the details ofthis process are described in relation to alignment of the second member66. The following description of the alignment of the first chosenmember 60 is presented in summary form and the details may be found inthe corresponding description of the alignment of the second chosenmember 66 which follows. The first set of alignment marks 64 on thefirst chosen member 60 is exposed to a flood of incoherent radiation 70to produce reflected radiation which passes through a lens 76 and isimaged on one surface 78 of an imaging detector means 80. The imagingdetector means 80 produces a serial electronic detection signal as afunction of time, which is input via line 81 to a special signalprocessing means 82 and then via line 85 to a comparator means 86 forcomparison with a reference signal produced by a reference signalgenerator means 84 in the same manner as described in relation toFIG. 1. The generation of an error signal by the comparator means andthe subsequent movement of the member 60 relative to the imagingdetector means 80 by the stage controller 88 via line 94 and thetransducer means 89 via line 95 are performed in a manner analogous tothat discussed in relation to FIG. 1, to achieve alignment of the member60 with the imaging detector means 80.

After the first chosen member 60 has been aligned with the imagingdetector means 80, the second chosen member 66, which is an opaquemember or a member with an opaque pattern formed thereon, such as a maskwhich has formed thereon the set of alignment marks 68 as well as apattern 67 required for the processing step to be performed subsequentto alignment in accordance with the process of the present invention, isplaced above and spaced at a predetermined, relatively small distancefrom the first chosen member 60. The distance between the first member60 and the second member 66 is chosen to be compatible with the depth offield of the lens in the imaging detector, which is discussed in moredetail below. On the surface of the second chosen member 66, there isformed a second set of alignment marks 68 which are substantiallyidentical to the set of alignment marks 64. The illustration in FIG. 4shows an alignment mark set of the type described in FIG. 2. Theperiodicity of the alignment marks 64 and 68 are typically chosen to beequal to the minimum video resolution element of the imaging detectormeans 80, in order to maximize the alignment accuracy.

A source of incoherent radiation 72 is located above and to one side ofthe second chosen member 66 at a predetermined distance therefrom and isused to produce the flood radiation 70, comprising, for example, lightradiation in the ultraviolet to far infrared wavelength range. The floodradiation 70 passes through the second chosen member 66 and strikes thesecond set of alignment marks 68. A flood pattern 74 of reflectedprimary radiation from the second set of alignment marks 68 is reflectedtoward a lens member 76 located within the imaging detector means 80.The lens 76 forms an image on one selected surface 78 of the imagingdetector means 80. The imaging detector means 80 receives the reflectedflood pattern in parallel, but reads this pattern serially in time togenerate a serial electronic detection signal as a function of time andcontaining serial information corresponding to the position of each markin the set of alignment marks 68.

More specifically, the imaging detector means may be a vidicon whichcomprises, for example, a magnifying lens system 76, a photoconductorsurface 78, a means for converting a photon picture to an electroncharge picture, and a means for scanning an electron beam across theelectron-charged surface to generate an electrical signal output. Animage of the set of alignment marks 68, comprising a pattern of lightand dark areas, is focused and magnified by the lens 76 onto the surface78, which comprises a material (such as an array of p-n junctions formedon a silicon substrate) whose conductivity is affected by photons. Theportions of the surface 78 exposed to photons have their conductivitychanged, while the portions of the surface 78 not exposed to photons donot have their conductivity altered. Thus, at the surface 79, there isformed a pattern of electron charges which corresponds to the pattern ofthe radiation striking the surface 78, which, in turn, corresponds tothe pattern of the set of alignment marks 68. A beam, such as anelectron beam, is scanned across the surface 79 having the electroncharge pattern to produce a serial electronic signal corresponding tothe electron charge pattern and thus corresponding to the pattern of theset of alignment marks 68.

The electronic signal produced by the imaging detector means 80 is aserial electronic signal having the same characteristics as thedetection signal discussed in relation to FIG. 1. This detection signalis input via line or conductor 81 to the special signal processing means82 which comprises narrow bandpass filtering and tuned amplificationmeans. The special signal processing means 82 generates an alignmentsignal which is subsequently input via line 85 to the comparator means86 for comparison to the reference signal generated by the referencesignal generator means 84 and input to the comparator means 86 via line87, in a manner analogous to that described in relation to FIG. 1. Thereference signal frequency and phase are predetermined as discussed inrelation to FIG. 1. The beginning of the reference signal issynchronized with the beginning of the deflection signal (not indicated)from the electron beam scan generator which is part of the imagingdetector means 80, as indicated by the legend on signal path 81 in FIG.4. Then, the frequency of the reference signal is electronicallyadjusted to equal the frequency of the alignment signal, as indicated bythe legend on signal path 83 in FIG. 4.

An error signal which is indicative of the extent of misalignment of thesecond chosen member 66 with a chosen fixed reference point, namely, thescan field of the imaging detector means 80, is generated by thecomparator means 86. The error signal is subjected to low-pass filteringand is input via line 91 to a means for producing the appropriatelateral movement of the second chosen member 66 to produce alignmentthereof with the scan field of the imaging detector means 80. Forexample, the error signal may be input to the stage controller means 88which generates drive signals via line 92 to the transducer means 90 tocause mechanical motion of a stage (not shown) holding the member 66,via line 93, in a manner similar to that described in relation toFIG. 1. Since the second chosen member 66 and the first chosen member 60have been accurately aligned with respect to the same fixed referencepoint (i.e. the scan field of the imaging detector means 80), themembers 60 and 66 are accurately aligned with each other. It should benoted that during such an alignment process, the imaging detector means80 must be maintained in a stable location.

Alternatively, in this second process embodiment of the presentinvention, the imaging detector means 80 shown in FIG. 4 may be a solidstate camera which comprises, for example, a magnifying lens system 76,an array of photodetector elements 78, a capacitor associated with eachphotodetector element, and a charge-coupled device for addressing eachcapacitor. The functioning of charge transfer devices and their use inarea image sensors, such as solid state video cameras, is described, forexample, by Sequin and Tompsett in the book entitled "Charge TransferDevices", Academic Press, New York, 1975, Chapters I, II and V. Suchsolid state cameras are commercially available, for example, a GeneralElectric's Solid-State Video Cameras Series TN2000, which may beobtained from the Optoelectronic Systems Operation of the GeneralElectric Company at Syracuse, N.Y. Briefly summarized, an image of theset of alignment marks 68, comprising a pattern of light and dark areasin the reflected radiation 74, is focused and magnified by the lens 76onto the array of photodetector element 78. Each photodetector elementcomprises a photosensitive area whose conductivity is affected byphotons, which changes the impedance of the former or generates avoltage. The photodetector elements exposed to photons have theirimpedance changed while the photodetector elements not exposed tophotons have their impedance unaltered. This electronic information fromeach photodetector element is stored in a separate capacitor. Then eachcapacitor is addressed by means of a charge-coupled device in bucketbrigade fashion, by which the level of an oscillating potential or clockis phased so that the charge from one capacitor is transferred to theadjacent capacitor and finally produces an output cell-by-cell. Thus,there is produced a serial electronic signal corresponding to thepattern of sensitivity in the photodetector array, which, in turn,corresponds to the pattern of reflected radiation from the set ofalignment marks 68. Consequently, in accordance with this alternativeprocess embodiment of the present invention, there is generated from theset of alignment marks a serial electronic detection signal containingserial information corresponding to the relative position of each markin the set of alignment marks.

This serial electronic detection signal from the solid state cameraimaging detector means is subsequently processed and used as describedabove with respect to the vidicon imaging detector means except that thereference point is different. Using a solid state camera, the fixedreference point is a predetermined element of the photodetector array,for example, the first element, which is visually sighted and countedfrom the upper left edge of the array. In addition, using a solid statecamera, the beginning of the reference signal is synchronized with thebeginning of the clocking out of the data stored in the capacitors asdescribed above. Thus, by this alternative embodiment of the presentinvention, the required serial electronic signal is generated withoutthe use of a scanning beam. One major advantage of the latter is thatthe stability of the alignment system is improved since the stability ofthe system depends on the mechanical stability of the detector array inits mounting, rather than on more easily variable electrical parameters,such as voltage, current, and stray magnetic fields, to which a scanningbeam is subject. In addition, a solid state camera has high reliabilityand is more straightforward to use than a complex scanning beam system.The solid state camera may be used in place of the vidicon as theimaging detector means in the various applications of the second andthird process embodiments of the present invention discussed in detailbelow.

In practicing the second process embodiment of the present invention,displacement of either of the alignment mark patterns by a distanceequal to the period of the pattern spacing would produce ambiguity indetermining the exact location of the alignment marks. Thus, it isnecessary to include a means for achieving coarse alignment of each ofthe sets of alignment marks 64 and 65 as a preliminary procedure, asdiscussed in relation to FIGS. 2 and 3.

Alternatively, by this second process embodiment of the presentinvention, alignment marks with a selected coded periodicity may be usedto generate corresponding detection and alignment signals in theselected code. The reference signal generated has a corresponding codeand the alignment and reference signals are correlated to generate anerror signal indicative of the extent of correlation of the alignmentand reference signals in time, as discussed in more detail in relationto FIG. 1.

Optionally, instead of comparing the detection signals from each ofmembers 60 and 66 with a reference signal and aligning each memberindependently as described above, the detection signals from each ofmembers 60 and 66 may be directly compared to each other to generate anappropriate error signal. More specifically, the alignment signalproduced from the set of alignment marks 64 may be used in place of thereference signal, for comparison to the alignment signal produced fromthe set of alignment marks 68. An error signal indicative of the extentof misalignment of the two sets of alignment marks 64 and 68 isgenerated as a result of this comparison and one of the members 60 or 66is moved in order to achieve alignment, as previously discussed.

In practicing the present invention in accordance with the secondprocess embodiment just described, and using the set of alignment marksas illustrated in FIG. 2, the first chosen member 60 is placed on theplatform 62 and a coarse alignment of the member 60 with the imagingdetector means 80 to within one period of the spacing between the marksin the first set of alignment marks 64 is performed as discussed inrelation to FIG. 2. The source of flood radiation 72 is used to producea flood of radiation, such as light in the visible to infraredwavelength range, which strikes the first set of alignment marks 64.Light reflected from the first set of alignment marks 64 is imaged bythe lens 76 onto the photoconductive surface 78 of the imaging detectormeans 80. The parallel information received by the imaging detectormeans 80 is read serially, to produce a serial electronic signal as afunction of time and containing serial information corresponding to theposition of each mark in the alignment mark set 64. This serialelectronic signal, or detection signal, is subjected to special signalprocessing, including narrow bandpass filtering and amplification, togenerate a fundamental periodic signal of specified frequency which isused as an alignment signal. The alignment signal is compared in thecomparator means 86 to the reference signal generated by the referencesignal generator means 84, to produce an error signal which isproportional to the phase difference of the alignment signal and thereference signal. The error signal is used to cause lateral movement ofthe first chosen member 60 until the alignment signal and referencesignal become congruent in phase, indicating accurate alignment of thefirst set of alignment marks 64 on the chosen member 60 with the chosenreference point (i.e. the scan field of the imaging detector means 80).

Next, the second chosen member 66 is placed in position as indicated inFIG. 4 and a coarse alignment of the member 66 with the imaging detectormeans 80 to within one period of the spacing between the marks in thesecond set of alignment marks 68 is performed in the manner discussedabove in relation to the coarse alignment of the member 60. Theprocedure described above for aligning the first chosen member 60 withrespect to the imaging detector means is repeated using the secondchosen member 66 and the second set of alignment marks 68 as the targetfor the flood radiation. Detection, alignment, and error signals areproduced as discussed above. Finally, the second chosen member 66 ismoved laterally with respect to the imaging detector means 80 until thealignment signal and reference signal become congruent in phase,indicating accurate alignment of the second set of alignment marks 68 onthe second chosen member 66 with the chosen reference point (i.e. theimaging detector means 80). Since both the first and second chosenmembers have been accurately aligned with respect to the same referencepoint, the first chosen member 60 is accurately aligned with the secondchosen member 66.

Thus, by this second process embodiment of the present invention justdescribed, it is possible to very accurately align two chosen memberswith respect to each other, to a fraction of a resolution element. Thisaccurate alignment and high resolution of the second process embodimentof the present invention are achieved in a manner similar to thatdiscussed in relation to FIG. 1, by generating serial electronic signalsas a function of time which have a high signal content and which containserial information corresponding to the position of each of the two setsof alignment marks. These serial signals are then optimized by narrowbandpass filtering to remove noise with negligible loss of signal. Thealignment signals so produced have a high signal-to-noise ratio whichenables highly accurate alignment to be achieved, as discussed in moredetail in relation to FIG. 1. In addition, the localized effects ofsurface defects in the alignment marks is minimized by this secondprocess embodiment of the present invention by virtue of the distributednature of the alignment information generated.

By a third process embodiment of the present invention, the first chosenmember 60 and the second chosen member 66 may be aligned with respect toeach other using simultaneous generation of the detection signal. Thesystem shown and described in relation to FIG. 4 may be used except thata different special signal processing means 82 is used. The secondchosen member 66 is selected to be a material which is transparent tothe flood radiation 70 so that this flood radiation 70 may pass throughthe second chosen member 66 and strike the first chosen member 60 whenthe two members are positioned as shown in FIG. 4. The two sets ofalignment marks 64 and 68 are selected to have different periodicities.For example, the set of alignment marks 64 may have the fine dimensionsdescribed in relation to FIG. 2 and the set of alignment marks 68 mayhave coarser dimensions and lower periodicity.

When the flood radiation 70 strikes the second chosen member 66, part ofthe radiation is reflected to the imaging detector means 80 in a patternimaging the set of alignment marks 68 and part of the radiation passesthrough the second chosen member 66 and strikes the first chosen member60 and the set of alignment marks 64. The radiation reflected from theset of alignment marks 64 passes through the member 66 and images theset of alignment marks 64 in the imaging detector means 80 as discussedin detail above. The detection signal produced by the imaging detectormeans 80 comprises a composite of two sine waves of differingperiodicity superimposed one on the other. The detection signal is inputvia line 81 to the special signal processing means 82 and is subjectedto special signal processing of the general type described in relationto other process embodiments of the present invention, including specialfiltering and amplification at a selected frequency. However, twofilters set at two different predetermined frequencies, are used inorder to separate the composite detection signal into its twocomponents, and to provide a first sine wave alignment signal of oneperiodicity representing the set of alignment marks 64 and a second sinewave alignment signal of a second periodicity representing the set ofalignment marks 68. Each sine wave alignment signal is then input vialines 85 to a separate comparator, such as comparator means 86, forcomparison to a reference signal of corresponding frequency generated bya reference signal generator means, such as reference signal generatormeans 84, and input via line 87 to the comparator means 86, to generatean error signal in a manner analogous to that described in relation toFIG. 1. Each reference signal frequency and phase are predetermined asdescribed in relation to FIG. 1. The beginning of each reference signalis synchronized with the beginning of the deflection signal from theelectron beam scan generator which is part of the vidicon meanspreviously discussed in detail, as indicated by the legend on signalpath 81 shown in FIG. 4. Then, the frequency of each reference signal iselectronically adjusted to equal the frequency of the correspondingalignment signal, as indicated by the legend on signal path 83 in FIG.4.

Next, the first and second chosen members are moved in response to thecorresponding error signal as discussed in relation to the secondembodiment of the present invention, until the corresponding alignmentand reference signals are congruent, indicating that the chosen memberis aligned with the fixed reference point--i.e. the beginning of thescan field in the imaging detector means or vidicon. In actual practice,the first chosen member 60 remains stationary and the electron beam scanfield of the vidicon is deflected to achieve alignment thereof with themember 60. Then, the chosen member 66 is moved by using a stagecontroller and transducer means until member 66 is aligned with the beamscan field. Since the first and second members are thus accuratelyaligned with respect to the same reference point, the two members areaccurately aligned with each other.

A preliminary coarse alignment procedure is performed prior to this finealignment process by using a conventional alignment process such as thatdescribed in the prior art section herein, using a matched set ofalignment marks.

In practicing the present invention in accordance with the third processembodiment just described, using the set of alignment marks shown inFIG. 2, the first and second chosen members are placed in position asshown in FIG. 4 and a coarse alignment of the two members is performedby a conventional alignment process as discussed above. The source offlood radiation 72 is used to produce a flood of radiation which strikesboth the first set of alignment marks 64 and the second set of alignmentmarks 68. The light reflecting from both sets of alignment marks isimaged by the lens 76 onto the photoconductive surface 78 of the imagingdetector means 80. The parallel information received by the imagingdetector means 80 is read serially, to produce a serial electronicsignal as a function of time and containing serial informationcorresponding to the position of each mark in each of the alignment marksets 64 and 68. This serial electronic signal, or detection signal, issubjected to special signal processing to separate it into two componentparts, one sine wave alignment signal representing the first set ofalignment marks 64 and one sine wave alignment signal representing thesecond set of alignment marks 68. Each alignment signal is compared to acorresponding reference signal to generate an error signal indicative ofthe extent of misalignment of the chosen member with the chosenreference point (i.e. the scan field of the imaging detector means.)Relative lateral movement of each member is provided in response to thecorresponding error signal until each alignment signal and correspondingreference signal become congruent in phase, indicating accuratealignment of the chosen member with the chosen reference point and thusof the two chosen members with each other.

Thus, by this third process embodiment of the present invention, it ispossible to very accurately align two chosen members with respect toeach other, to a fraction of a resolution element. This accuratealignment and high resolution are achieved by generating a compositeserial electronic signal as a function of time which contains serialinformation indicative of the position of each mark in each of the twosets of alignment marks. This composite signal is then subjected tospecial signal processing to separate the composite signal into twocomponent alignment signals and to optimize the alignment signals toproduce unambiguous signals with high signal-to-noise ratios, whichenable highly accurate alignment to be achieved.

As an option to the third process embodiment of the present inventionjust described, the alignment mark sets 64 and 68 need not besuperimposed one on the other. Instead, the alignment mark sets 64 and68 may be positioned to be side-by-side in different horizontal planesand with ends of the marks in one alignment mark set facing the ends ofthe marks in the second alignment mark set. The detection signalgenerated by the imaging detector means contains serial informationcorresponding to the position of one set of alignment marks in the firsthalf of the signal and serial information corresponding to the positionof the second set of alignment marks in the second half of the signal.The detection signal is electronically processed to produce acorresponding two-component alignment signal. The first half of thealignment signal is compared to a reference signal of predeterminedfrequency and phase to generate a first error signal which is used tocause relative lateral movement of the first chosen member to produceaccurate alignment thereof with the fixed reference point (i.e. the scanfield of the imaging detector means). The second half of the alignmentsignal is compared to the same reference signal to generate a seconderror signal which is used to cause relative lateral movement of thesecond chosen member to produce accurate alignment thereof with thefixed reference point and thus with the first chosen member. (In someinstances, it may be necessary to use two separate reference signals forthe two halves of the alignment signal.)

FIG. 5 shows, in schematic form, the detection and alignment signalsgenerated in accordance with one embodiment of the present invention,using the system shown in FIG. 1 and the alignment mark shown in FIG. 2.The sine wave A shown in FIG. 5 represents the alignment signal producedafter the special signal processing in accordance with the presentinvention as discussed in relation to FIG. 1. The waveform B shown inFIG. 5 represents the detection signal produced in accordance with thepresent invention, from which the alignment signal A is formed byspecial signal processing. The waveform B comprises the sine wave A withnoise of height N superimposed thereon. The detection signal isdescribed by Equation (2) below. ##EQU2## where y=detection signal

S=height of sine wave

Δ=period of grating alignment mark pattern

X=distance along the plane being scanned

The uncertainty in the position of the alignment signal with respect tothe reference signal can be estimated from the uncertainty in theposition of the zero crossing, δ, of the "noisy" sine wave. Thisuncertainty in zero crossing, δ, is related to the noise amplitude shownin Equation (3) below. ##EQU3## where N=noise amplitude

δ=uncertainty at zero crossing

S=height of sine wave

δ=period of grating of alignment mark pattern

From Equation (3), the value of δ may be derived as shown in Equation(4) below: ##EQU4## Given a value for S/N of 7 and for Δ of 0.2micrometers, δ may be calculated from Equation (4) to be ±45 angstroms.Thus, the required alignment accuracy of 0.1 of the beam diameter can beeasily attained in this specific illustration by the process of thepresent invention.

EXAMPLE 1

In practicing the present invention using the system described in FIG.1, an alignment target of the type described in FIG. 2 was formed in awafer of bare silicon having dimensions of 1 inch by 1 inch. Thealignment target comprised 24 bars, each having a width of 0.15micrometer, a length of 100 micrometers and a depth of 100 to 1000angstroms, and the space between adjacent bars was approximately 0.4micrometer. The dimensions of the total set of alignment marks was 10micrometers by 10 micrometers. The focused beam comprised a beam ofgallium ions, at an energy of 60 kiloelectronvolts and a fluence of 1ampere per centimeter², focused to a spot size of 1500 Å. The focusedion beam was scanned at a velocity approximately 6.8 millimeters persecond. The special signal processing was performed by a spectrumanalyzer (Hewlett-Packard, model 3580A) and the alignment signal wasdisplayed on the spectrum analyzer. A predominant peak at 17 kilohertzwas obtained and represented the sinusoidal component of the barpattern. From the frequency spectrum display of the spectrum analyzer,the signal-to-noise ratio was determined to be 7. The potentialalignment precision was then calculated using Equation (4) above and wasfound to be ±90 angstroms.

An alignment process of the type described using a focused ion beamfinds utility in such semiconductor device fabrication processes as ionimplantation to form doped regions in a substrate, ion beam milling oretching to directly structure a substrate or layer deposited thereon,and pattern generation for forming a mask structure.

EXAMPLE 2

In practicing the present invention in accordance with the secondprocess embodiment described in relation to FIG. 4, an alignment targetof the type described in FIG. 2 is formed on the surface of a baresilicon wafer and a matching set of alignment marks is formed on thesurface of a mask member located above the wafer. The target comprises24 bars each having a width of 10 micrometers, a length of 1 millimeter(mm), and a depth of 1 micrometer, and spaced from adjacent bars by adistance of approximately 20 micrometers. The dimensions of the totalset of alignment marks are 1 mm by 1 mm. In addition, a coarse alignmentbar is formed adjacent the fine alignment bar pattern. The radiationconsists of incoherent radiation having wavelengths within the visiblerange. The imaging detector means used is a solid state camera (i.e.using charge-coupled devices). The special signal processing isperformed by a spectrum analyzer (e.g. Hewlett-Packard, Model 3580A).

Alignment processes of the type described in relation to the second andthird process embodiments of the present invention find utility, forexample, in processes for making microminiaturized semiconductor devicesin which a pattern from a mask is replicated in a substrate or layerformed thereon, such as during ion implantation, etching, or depositinga patterned layer on a substrate.

While the present invention has been particularly described with respectto the preferred embodiments thereof, it will be recognized by thoseskilled in the art that certain modifications in form and detail may bemade without departing from the spirit and scope of the invention. Inparticular, the scope of the invention is not limited to the use of abeam of ions or of light waves, which were used merely as examples, butmay include any radiation which is capable of producing secondaryemissions (i.e. reflected primary radiation or secondary radiation) fromthe alignment target as described herein. In addition, the presentinvention is not limited to the particular signal detection andprocessing means described herein, but is intended to include any meansfor generating a periodic or encoded serial electronic signal as afunction of time and containing serial information corresponding to theposition of each mask in the set of alignment marks.

Further, the present invention is not limited to the special signalprocessing operations described herein but is intended to include anymeans for generating a periodic signal at a selected frequency which maybe used for the purpose of very accurate alignment. In addition, themeans for moving one chosen member with respect to another or withrespect to a focused beam is not limited to the mechanical means or beamdeflection means discussed herein, but includes any means for providingsuch motion in response to an error signal.

Moreover, the novel process and system concepts set forth herein are notdependent on or limited to the particular functional electronics of thevarious stages or blocks illustrated in FIGS. 1 and 4. The functions andcapabilities of each stage shown in FIGS. 1 and 4, such as a comparator,a detector, a reference signal generator, and a tuned amplifier, arewell-known to those skilled in the art and are commercially available,and any such suitable electronics means known to those skilled in theart may be used in practising the present invention.

Further, the present invention is not limited to the alignment of awafer of a semiconductor material, but may be used for any materialwhich is compatible with the alignment process described herein.Finally, it is not intended to limit the scope of the present inventionto processes for manufacturing microminiaturized devices and circuits,which was used merely as an example, but rather to include any processin which a chosen member is aligned with a focused beam or in which afirst chosen member is aligned with a second chosen member as, forexample, in target acquisition in missile systems or in precisionmachining procedures.

What is claimed is:
 1. A process for aligning the surface of a chosenmember with a chosen fixed reference point comprising the steps of:(a)providing a set of alignment marks on said surface of said member andspaced at a predetermined periodicity; (b) providing said chosen fixedreference point; (c) providing a source of incoherent flood radiationfor illuminating said set of alignment marks; (d) illuminating said setof alignment marks with said flood radiation to produce reflectedradiation therefrom; (e) imaging said reflected radiation onto animaging detector means comprising a charge-coupled device to generatefrom said set of alignment marks a detection signal comprising a serialelectronic signal as a function of time and containing serialinformation corresponding to the position of said set of alignmentmarks; (f) electronically processing said detection signal to generatean alignment signal with a high signal-to-noise ratio; (g) providing areference electrical signal comprising a periodic signal of apredetermined frequency and phase; (h) comparing the phase angle of saidalignment signal and the phase angle of said reference signal andgenerating an error signal proportional to the phase difference of saidalignment signal and said reference signal; and (i) providing lateralmovement of said set of alignment marks with respect to said chosenreference point in response to said error signal until said alignmentsignal and said reference signal become congruent in phase, to produceaccurate alignment of said surface of said chosen member with saidchosen reference point.
 2. A process for aligning the surface of a firstchosen member with the surface of a second chosen member which includesthe steps of:(a) providing a first set of alignment marks on saidsurface of said first member and spaced at a predetermined periodicity;(b) providing a chosen fixed reference point for determining therelative positions of said first and said second chosen members; (c)providing a source of incoherent flood radiation for illuminating saidfirst set of alignment marks; (d) illuminating said first set ofalignment marks with said flood radiation to produce first reflectedradiation therefrom; (e) imaging said first reflected radiation onto animaging detector means comprising a charge-coupled device to generate afirst serial electronic signal as a function of time and containingserial information corresponding to the position of said first set ofalignment marks; (f) electronically processing said first detectionsignal to generate a first alignment signal with a high signal-to-noiseratio; (g) providing a first reference electrical signal comprising aperiodic signal of a predetermined frequency and phase; (h) comparingthe phase angle of said first alignment signal and the phase angle ofsaid first reference signal and generating a first error signalproportional to the phase difference of said first alignment signal andsaid first reference signal; (i) providing lateral movement of saidfirst set of alignment marks with respect to said chosen reference pointin response to said first error signal until said first alignment signaland said first reference signal become congruent in phase, to produceaccurate alignment of said surface of said first chosen member with saidchosen reference point; (j) providing a second set of alignment marks onsaid surface of said second member and which is substantially identicalto said first set of alignment marks; (k) illuminating said second setof alignment marks with said flood radiation to produce second reflectedradiation therefrom; (l) imaging said second reflected radiation ontosaid imaging detector means comprising a charge-coupled device togenerate a serial electronic signal as a function of time and containingserial information corresponding to the position of said second set ofalignment marks; (m) electronically processing said second detectorsignal to generate a second alignment signal with a high signal-to-noiseratio; (n) providing a second reference electrical signal comprising aperiodic signal of predetermined frequency and phase; (o) comparing thephase angle of said second alignment signal and the phase angle of saidsecond reference signal and generating a second error signalproportional to the phase difference of said second alignment signal andsaid second reference signal; and (p) providing lateral movement of saidsecond set of alignment marks with respect to said chosen referencepoint in response to said error signal until said second alignmentsignal and said second reference signal become congruent in phase, toproduce accurate alignment of said surface of said second chosen memberwith said chosen reference point and thus produce accurate alignment ofsaid surface of said first chosen member with said surface of saidsecond chosen member.
 3. A process for aligning the surface of a chosenmember with a chosen fixed reference point comprising the steps of:(a)providing a set of alignment marks on said surface of said member andspaced at a selected coded periodicity; (b) providing said chosen fixedreference point; (c) providing a source of incoherent flood radiationfor illuminating said set of alignment marks; (d) illuminating said setof alignment marks with said flood radiation to produce reflectedradiation therefrom; (e) imaging said reflected radiation onto animaging detector means comprising a charge-coupled device to generatefrom said set of alignment marks a detection signal comprising a serialelectronic signal as a function of time and containing serialinformation corresponding to the position of said set of alignmentmarks; (f) electronically processing said detection signal to generatean alignment signal with a high signal-to-noise ratio; (g) providing areference electrical signal comprising an encoded signal with codingcorresponding to said selected coded periodicity of said set ofalignment marks; (h) correlating said alignment signal and saidreference signal and generating an error signal which is indicative ofthe extent of correlation of said alignment signal and said referencesignal in time; and (i) providing lateral movement of said set ofalignment marks with respect to said chosen reference point in responseto said error signal until said alignment signal and said referencesignal become correlated, to produce accurate alignment of said surfaceof said chosen member with said chosen reference point.